Representative of the background in the field of dosing detergents in powdered form is British patent specification GB-A 2 111 946, in which a dispensing device for detergents in liquid or powdered form is disclosed. As a step in a programmed cleaning process a powdered detergent is caused to flow from a supply container through an opened valve into a metering cup positioned thereunder. When a desired quantity of the powdered detergent, corresponding to the volume of the metering cup, is to be dispensed for addition to a washing water flow, a delivery valve in the bottom of the metering cup is opened and simultaneously the discharge valve in the lower end of the supply container is closed. In case particulate material is used it is pronosed to vibrate the supply container in order to promote the flow of material within and from the supply container. In this way the tendency of powdered material to arc within the supply container could be opposed indeed. When closing the outlet valve of the supply container, however, a quantity of powder may get caught between the conically shaped valve body and the correspondingly shaped outlet portion of the supply container and get compressed. As a result of this the said outlet valve would not close correctly and the delivery valve of the metering cup , mchanically coupled with said outlet valve, would not open sufficiently, so that the outlet flow from the metering cup would not be satisfactory. Consequently, the manner of dosing as disclosed in patent specification GB-A 2 111 946 would not be suitable for material in powdered form. Another disadvantage of the described batchwise dosing--also called "chamber dosing", is that the dosing volume cannot be adjusted in a simple way. Therefore, in modern fully automatized cleaning processes detergents are hitherto dosed in the liquid state in which the ultimate washing process is taking place. As an example of this "direct dosing" reference is made to the International Patent Application WO 91/12763 and to the European patent document EP-A 0 611 159.
Another example is the dosing of powdered material in the brewing process of drinks, such as coffee, chocolate and the like in drink dispensing machines, wherein powdered material is dosed by means of a rotating screw conveyor, which is supplied from a hopper and is caused to make a number of revolutions, that corresponds with the desired metering amounts.
The powdered material to be dosed is often rather hygroscopic, so that the presence of moisture may lead to the material becoming caked within the screw conveyor; the dosing may even become completely "blocked". On the other hand undesired "afterflow" of powdered material often takes place, either spontaneously or as a result of vibrations transferred to the metering screw.
Attempts have already been made to meet problems connected to dosing of powdered materials.
A proposed solution is disclosed in U.S. Pat. No. 2,865,537, dating from 1958. Powdered material, such as a detergent is proposed to be metered from a hopper having a discharge tube, by squeezing the discharge tube at places, which periodically move along a tube section of a certain length. The squeezing of the discharge tube is proposed to be effected by means of a set of rotors positioned on opposite sides of the vertically suspending discharge tube and mounted for rotation in opposite directions about horizontal axes, each of said rotors comprising a plurality (sixth) of rollers that are evenly spaced according to a common circumscribed cylinder. The two rotors are placed "out of phase" and radially spaced such, that each time a roller of one rotor projects between two rollers of the second rotor. As seen in a cross-sectional plane perpendicular to the rotor axis, the upstream portion of the discharge tube is deformed by the cooperating rollers to a downstream moving funnel which is closed at its downstream end. According to said funnel travelling further in the downstream direction, it becomes increasingly narrower until the moment, at which the volume of the funnel has in fact been reduced to zero, due to the opposite funnel walls having become superposed and stretched between a roller of one rotor and a roller of the second rotor.
It will be understood, that during travelling of the initially wide funnel in the downstream direction, powdered material tends to be expelled from the narrowing funnel back into the upstream direction. Finally, subject to the type of powdered material and to the degree of elasticity of the wall of the discharge tube, no more than a thin film of the material may collect between the superposed funnel walls. Thereupon, while the superimposed funnel walls are allowed to spread and form a discharge funnel, that is widening in the downstream direction, said thin film of powdered material may easily stick to the funnel walls. Consequently there is in fact no question of dosing in this case.
Now the invention aims at providing a better and more effective solution.